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Line 443... |
and placed right before the actual application code so it gets executed right after reset.
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and placed right before the actual application code so it gets executed right after reset.
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The `crt0.S` start-up performs the following operations:
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The `crt0.S` start-up performs the following operations:
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[start=1]
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[start=1]
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. Initialize all integer registers `x1 - x31` (or jsut `x1 - x15` when using the `E` CPU extension) to a defined value.
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. Initialize all integer registers `x1 - x31` (or just `x1 - x15` when using the `E` CPU extension) to a defined value.
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. Initialize the global pointer `gp` and the stack pointer `sp` according to the `.data` segment layout provided by the linker script.
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. Initialize the global pointer `gp` and the stack pointer `sp` according to the <<_ram_layout>> provided by the linker script.
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. Initialize all CPU core CSRs and also install a default "dummy" trap handler for _all_ traps. This handler catches all traps during the early boot phase.
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. Initialize all CPU core CSRs and also install a default "dummy" trap handler for _all_ traps. This handler catches all traps
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. Clear IO area: Write zero to all memory-mapped registers within the IO region (`iodev` section). If certain devices have not been implemented, a bus access fault exception will occur. This exception is captured by the dummy trap handler.
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during the early boot phase.
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. All interrupt sources are disabled and all pending interrupts are cleared.
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. Clear all counter CSRs and stop auto-increment.
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. Clear IO area: Write zero to all memory-mapped registers within the IO region (`iodev` section). If certain devices have not
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been implemented, a bus access fault exception will occur. This exception is captured by the dummy trap handler.
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. Clear the `.bss` section defined by the linker script.
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. Clear the `.bss` section defined by the linker script.
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. Copy read-only data from the `.text` section to the `.data` section to set initialized variables.
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. Copy read-only data from the `.text` section to the `.data` section to set initialized variables.
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. Call the application's `main` function (with _no_ arguments: `argc` = `argv` = 0).
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. Call the application's `main` function (with _no_ arguments: `argc` = `argv` = 0).
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. If the `main` function returns `crt0` can call an "after-main handler" (see below)
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. If the main function returns...
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. If there is no after-main handler or after returning from the after-main handler the processor goes to an endless sleep mode (using a simple loop or via the `wfi` instruction if available).
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** the return value is copied to the <<_mscratch>> CSR to allow inspection by the on-chip debugger.
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** an optional <<_after_main_handler>> is called (if defined at all).
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** the last step the CPU does is entering endless sleep mode (using the `wfi` instruction).
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:sectnums:
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:sectnums:
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===== After-Main Handler
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===== After-Main Handler
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If the application's `main()` function actually returns, an _after main handler_ can be executed. This handler can be a normal function
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If the application's `main()` function actually returns, an _after main handler_ can be executed. This handler is a "normal" function
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since the C runtime is still available when executed. If this handler uses any kind of peripheral/IO modules make sure these are
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as the C runtime is still available when executed. If this handler uses any kind of peripheral/IO modules make sure these are
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already initialized within the application or you have to initialize them _inside_ the handler.
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already initialized within the application. Otherwise you have to initialize them _inside_ the handler.
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.After-main handler - function prototype
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.After-main handler - function prototype
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[source,c]
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[source,c]
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----
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----
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int __neorv32_crt0_after_main(int32_t return_code);
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void __neorv32_crt0_after_main(int32_t return_code);
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----
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----
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The function has exactly one argument (`return_code`) that provides the _return value_ of the application's main function.
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The function has exactly one argument (`return_code`) that provides the _return value_ of the application's main function.
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For instance, this variable contains _-1_ if the main function returned with `return -1;`. The return value of the
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For instance, this variable contains `-1` if the main function returned with `return -1;`. The after-main handler itself does
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`__neorv32_crt0_after_main` function is irrelevant as there is no further "software instance" executed afterwards that can check this.
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not provide a return value.
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However, the on-chip debugger could still evaluate the return value of the after-main handler.
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A simple UARt output can be used to inform the user when the application's main function returns
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A simple UART output can be used to inform the user when the application's main function returns
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(this example assumes that UART0 has been already properly configured in the actual application):
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(this example assumes that UART0 has been already properly configured in the actual application):
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.After-main handler - simple example
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.After-main handler - simple example
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[source,c]
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[source,c]
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----
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----
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int __neorv32_crt0_after_main(int32_t return_code) {
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void __neorv32_crt0_after_main(int32_t return_code) {
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neorv32_uart0_printf("\n main function returned with exit code %i. \n", return_code); <1>
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neorv32_uart0_printf("\n main function returned with exit code %i. \n", return_code); <1>
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return 0;
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}
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}
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----
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----
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<1> Use `` here to make clear this is a message comes from the runtime environment.
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<1> Use `` here to make clear this is a message comes from the runtime environment.
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